In a general definition, clock signals are repetitive digital signals, i.e. they are voltage waves toggling from a low logic state “0” to a high logic state “1”. The duty-cycle for these signals, being defined as the ratio between the time the signal is at logic state “1” over the repetition period or clock period, can be of any value between 0 and 1. In many practical applications, the duty-cycle is 0.5.
Clock signals are needed in electronic devices for a variety of different circuits, blocks, sub-systems or systems. Some examples include microprocessors or microcontrollers, DC-DC converters, switched capacitor filters and so on. In a limited number of cases, the clock period changes during time, for example with respect to different working phases of the corresponding device, while in the most common applications, the clock period is instead fixed and it does not vary over the time.
Moreover, for instance during a testing phase of an electronic device, it is often required to configure the clock period or clock phase among a range of possible values. Sometimes, it might be needed to finely adjust the clock period or clock phase to match a designed target. For example, it is sometimes needed to operate an electronic device at a different speed or to correct a clock period value or clock phase to compensate some fabrication process spread.
Digital oscillators or clock generators are electronic circuits used to generate digital clock signals. Amongst the different architectures of an oscillator, the ring oscillator is one of the most popular and simple.
An oscillator with temperature compensation and a clock output inhibition control is disclosed for instance in the U.S. Pat. No. 6,052,035.
In other cases, it may be also desirable to have different clock signals with a precise time difference between them. As an example, two signals with a time difference between their rising edge, same period and same duty cycle could be useful to generate different phase signals needed for a memory reading phase. For example, it may be a reading phase of a ROM or a RAM memory.